1. Field of the Invention
The present invention relates to an evoked response detector for a heart stimulator for determining evoked response in the presence of polarization, and to a heart stimulator having such an evoked response detector incorporated therein.
2. Description of the Prior Art
Cardiac stimulators are known which have a pulse generator devised for producing stimulation pulses of varying amplitudes, and a lead adopted to be introduced into the heart of a patient and connected to the pulse generator for delivering stimulation pulses to the heart, and an evoked response detector having measuring and memory means for measuring and storing the electrode signal picked up by the lead in response to delivered stimulation pulses, wherein at least one of said stimulation pulses has a sufficiently high amplitude for obtaining capture.
To reduce the energy consumption of heart stimulators a so called AUTOCAPTURE.TM. function is used to maintain the energy of the stimulation pulses at a level just above that which is needed to effectuate capture, cf. e.g. U.S. Pat. No. 5,458,623. A reliable detection of the evoked response, which then is necessary, is, however, not a simple matter, especially when it is desired to sense the evoked response with the same electrode as the one delivering the stimulation pulse. This is because of the fact that the evoked response potential could be small in amplitude compared to the residual polarization charge. The residual charge decays exponentially but tends to dominate the evoked potential for several hundreds of milliseconds after the stimulation. If the polarization is too high, it could be wrongly interpreted by the evoked response detector as a capture, i.e. contraction of the heart. The AUTOCAPTURE.TM. algorithm could then by mistake adjust the output amplitude of the stimulation pulse to a value below the actual capture level, which will result in no capture. If the electrode surface of the electrode lead in use has significant polarization this could consequently disturb the AUTOCAPTURE.TM. function and result in loss of capture. To guarantee a safe and reliable detection of evoked response it is thus desirable to use leads having electrode surfaces with low polarization.
Several attempts have been made to solve the lead polarization problems in connection with evoked response detection. Thus U.S. Pat. No. 5,417,718 discloses a system for maintaining capture wherein electrical post-stimulus signal of the heart, following delivery of a stimulation pulse, is compared to a polarization template, determined during a capture verification test. A prescribed difference between the polarization template and the post-stimulus signal indicates capture. Otherwise loss of capture is presumed and the stimulation energy is increased a predetermined amount to obtain capture.
There is mostly at least one significant slope in the bipolar measured IEGM signal, which makes it possible to discriminate the evoked response signal from slowly varying signals such as polarization signals. Thus in U.S. Pat. No. 5,431,693 a method of verifying capture of the heart by a cardiac pacemaker is described by observing that the non-capture potential is exponential in form and the evoked capture potential, while generally exponential in form, has one or more small-amplitude perturbations superimposed on the exponential wave form. These perturbations are enhanced for ease of detection by processing the wave form signal by differentiation to form the second derivative of the evoked response signal for analysis for the evoked response detection.
Unipolar detection of evoked response signals is, however, not possible using this technique. Abrupt slope changes or superimposed small-amplitude perturbations are leveled out if the measurements are made over a longer distance from the electrode to the stimulator casing.
Experiments have now shown that the evoked response signal amplitude is fairly constant, independent of the stimulation pulse amplitude, i.e. the evoked response signal amplitude does not vary with the amplitude of the stimulation pulse (provided that the stimulation amplitude is above the capture threshold) Further, it has been found that the electrode polarization is approximately linearly dependent on the stimulation pulse amplitude for a constant pulse duration. Experimental results are presented in greater detail below in connection with the description of FIGS. 1-3.